What Pete said.
Except, if kj_posts.ID is the primary key then it will be sufficient enough to group on kj_posts.ID alone and leave out the other columns as Pete listed as adding them to the query is being wasteful by asking MySQL to perform grouping on data which will result in no change to the results.
So Query in full:
sql code:
SELECT kj_posts.ID, kj_posts.post_date, max( kj_comments.comment_ID),
kj_posts.post_title FROM kj_posts LEFT JOIN kj_comments ON
kj_comments.comment_post_id = kj_posts.ID GROUP BY kj_posts.ID
ORDER BY kj_posts.post_date